1. Field of the Invention
The present invention relates to an optical head for recording information into an optical disk and reproducing information from the optical disk and an optical disk unit having the optical head. In addition, the present invention relates to a method for eliminating an effect of a spherical aberration caused by the thickness error of a surface resin layer of an optical disk.
2. Description of the Related Art
In an optical head and an optical disk unit, in order to improve efficiency of utilizing light, there has been proposed a method and apparatus for reducing an effect of spherical aberration caused by the thickness error of a surface resin layer (light transmission layer) of an optical disk.
For example, in Jpn. Pat. Appln. KOKAI Publication No. 11-259906, there is disclosed an optical head 1 for emitting light from a light transmission layer 4 in a recording layer provided between a substrate 3 and the light transmission layer 4, wherein a collimator lens 13 and an actuator 14 for a collimator lens are provided, and the collimator lens 13 provided between a light source 10 and an objective lens 16 is moved so as to offset a spherical aberration caused by the thickness error of the light transmission layer 4.
In Jpn. Pat. Appln. KOKAI Publication No. 11-259906, when the collimator lens 13 is moved in order to correct the spherical aberration caused by the thickness error of the light transmission layer 4, no consideration is taken into the fact that a light quantity on the objective lens 16 changes. This causes a problem in the case where light intensity is set during reproduction in particular.
That is, in order to correct a spherical aberration caused by the thickness error of a light transmission layer, when a collimator lens is moved in an optical axis direction, the light amount incident to an objective lens is changed. Thus, there is a problem that the quantity of light emitted from the objective lens changes during information reproduction, and a recording mark cannot be precisely recorded during information recording.
In the above described publication, there is disclosed the thickness of a light transmission layer by using a return light from the surface of the light transmission layer and a return light from an information recording layer.
In addition, in the optical disk in which a plurality of recording layers are provided, a distance between each recording layer and the light transmission layer, i.e., the thickness in light transmission layer differs from one another. Thus, there is a problem that information recording and reproduction is more unstable.